The atrioventricular (AV) junction is the heart of the embryonic heart, a central site where morphogenetic processes intersect to transform and align a tubular heart into a 4-chambered organ. Thus, it is not surprising that many human congenital heart defects can arguably be linked to AV junction formation and remodeling. For this revised, renewal application we continue to propose two aims directed at understanding the molecular mechanisms that regulate the assembly of multiple mesenchymal primordia into an AV valvuloseptal complex and its subsequent remodeling into a central fibrous body (or septum), AV septal valve leaflets and a fibrous continuity that anchors valves, aligns the ventricular inlet with their correct outlets and electrically insulates the atria from the ventricles. These aims wil test hypotheses related to the formation (Aim 1) and remodeling (Aim 2) of the AV valvuloseptal complex. We focus on the function of two distinct matricellular proteins, CCN1/cry61 and periostin, whose expression patterns and null phenotypes indicate key signaling roles in AV morphogenesis and remodeling. In Aim 1, we use CCN1 knockout mice with fully penetrant AV septal defects to test the hypothesis that CCN1 activates specific integrin signaling pathways in the AV progenitor cells that activate (phosphorylate) specific effector proteins associated with cell processes that drive formation, integration (fusion) and of the embryonic AV valvuloseptal complex. The effector proteins include hyaluronan synthetase2, caspase-3, and filamin-A which upon phosphorylation activate hyaluronan (HA) production, apoptosis and and filamin-A, an actin binding protein that regulates cytoskeletal functions. In Aim 2, we test the hypothesis using genetic models and 3D culture assays that periostin - secreted by endocardial, epicardial and CD45 derived AV valvuloseptal progenitor cells - interacts with specific integrins to activate signaling complexes that activate filamin-A. Using gene silencing vectors or vectors expressing mutated FLNA that cannot be activated we will test the significance of FLNA activation on the differentiation and maturation of the AV valvuloseptal complex. Also, as a first step in understanding the functional significance of f the AV septal complex being derived from progenitor cells of different lineages , we will conditionally delete filamin-A, a downstream targe of periostin signaling from each lineage under the hypothesis that integration or interaction of mesenchymal subtypes is required for formation and remodeling of AV primordia. Unique aspects of this proposal are that we for the first time the whole of the AV complex, not just individual parts, and perform signaling studies that have rarely been extended to cardiac development. PUBLIC HEALTH RELEVANCE: The proposed studies will identify the mechanisms by which matricellular proteins periostin and CCN1, based on their reciprocal expression patterns and null phenotypes, have key regulatory roles in the formation of the atrioventricular septal complex required for the AV junction remodelling into an adult heart phenotype.